Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
New Member

Null 0

what is null o for?

3 REPLIES
Hall of Fame Super Bronze

Re: Null 0

From

http://www.cisco.com/warp/public/471/route_to_null_interface.html

"The Null interface is typically used for preventing routing loops. Enhanced Interior Gateway Routing Protocol (EIGRP), for instance, always creates a route to the Null0 interface when it summarizes a group of routes. Whenever a routing protocol summarizes, this means that the router might receive traffic for any IP address within that summary. Because not all IP addresses are always in use, there is a risk of looping packets in case default routes are used on the router which receives the traffic for the summary"

---

Please rate helpful posts.

Thanks

New Member

Re: Null 0

In addition to previous post, another good use of null0 static route is to blackhole any routes to private addresses (10.x, 192.168, 172.16...) on your Internet routers. E.g. ip route 10.0.0.0 255.0.0.0 null0

Re: Null 0

hi...

any thing that is point to the null 0 will discarded... it means its bit bucket for discarding packet...

and how null0 will prevent the routing look just look at this:

You summarise the routes and advertise it WHY so that the routing table of the peer routers will have less entries in routin table and need not be populated with all routes which are in that summarised route.

Now WHAT happens if any of the nodes behind the peer routers try to reach any of the specific route in that summarised route it will be fwded to your distribution router on which you have summarised teh routes and from there it will look for most specific route from that summarised route NOW lets assume that specific route is down then WHAT will happen it will not find any entry in routing table and either will be dropped or incase you have a default route it will be fwded to that default router and from there again it will check the summarised route and will be fwded to the distribution router and this will go on and on and on.....so to stop this if you have a static route for summarised route pointing to null0 interface and if most specific route within that summarised route is down so any request for that destination will be fwded to null0 static route because that is the next best match and will be dropped.

So that is the intention behind this configuration to avoid possible routing loop.

hope this will help you

rate this post if it helps

regards

Devang

1924
Views
16
Helpful
3
Replies
CreatePlease to create content